1. Field of the Invention
The present invention relates to a liquid crystal device such as a display device, liquid crystal shutter, and so forth, for implementing stable operation by use of a liquid crystal panel, and in particular, to a liquid crystal device capable of implementing low power consumption by utilizing a memory effect owing to a bistable state of liquid crystals with a memory effect.
2. Description of the Related Art
Vigorous efforts have lately been exerted on research and development on a liquid crystal panel with a memory effect, represented by a ferroelectric liquid crystal device, and so forth, and the liquid crystal panel with a memory effect is being used in display devices, liquid crystal shutters, and the like by making the most of its feature of low power consumption. For example, International Publication No. WO 00/23848 A1 (a first embodiment, FIG. 11, and so forth) has disclosed a ferroelectric liquid crystal device capable of effecting excellent display by controlling a sticking phenomenon due to variations in threshold value of the ferroelectric liquid crystal device, and a method for driving the same. A conventional ferroelectric liquid crystal device is described hereinafter with reference to the accompanying drawings.
FIG. 11A is a schematic plan view showing a configuration of arrangement of polarizers of the ferroelectric liquid crystal device disclosed in the Patent Document described as above. In FIG. 11A, with a ferroelectric liquid crystal device 20, a liquid crystal layer 22 is disposed between a first polarizing film 21a and a second polarizing film 21b, arranged in crossed Nicols, as shown by a broken line. The liquid crystal layer 22 is disposed such that either a polarization axis A of the first polarizing film 21a, or a polarization axis B of the second polarizing film 21b is substantially parallel with a long axis direction of the molecule of the liquid crystal layer 22 at the time of liquid crystal molecules being in a first stable state (indicated by an arrow C), or in a second stable state (indicated by an arrow D). In the case of an example shown in FIG. 11A, the liquid crystal layer 22 is disposed such that the polarization axis A of the first polarizing film 21a is substantially parallel with the a long axis direction of the molecule at the time of the liquid crystal molecules being in the first stable state (indicated by the arrow C).
Next, FIG. 11B is a schematic sectional view showing the structure of the ferroelectric liquid crystal device 20. In FIG. 11B, the ferroelectric liquid crystal device 20 comprises the liquid crystal layer 22 composed of the liquid crystals with a memory effect having at least two stable states, and a pair of glass substrates 23a, 23b, with the liquid crystal layer 22 sandwiched therebetween. Further, the glass substrates 23a, 23b are fixedly attached to each other with a sealant 26. A plurality of scanning electrodes 24, and signal electrodes 25, serving as driving electrodes, are installed over respective surfaces of the glass substrates 23a, 23b, opposed to each other, and alignment layers 27a, 27b are vapor-deposited over the scanning electrodes 24, and the signal electrodes 25, respectively.
Further, the first polarizing film 21a is disposed on the outer side of the glass substrate 23a, on one side of the ferroelectric liquid crystal device 20, such that the polarization axis of the first polarizing film 21a, will be in parallel with the a long axis direction of the molecule of the liquid crystal layer 22 at the time of the liquid crystal molecules being in the first or second stable state while the second polarizing film 21b is disposed on the outer side of the glass substrate 23b, on the other side of the ferroelectric liquid crystal device 20, such that the polarization axis of the second polarizing film 21b will differ by 90 degrees in direction from the polarization axis of the first polarizing film 21a. 
Now, operation of the ferroelectric liquid crystal device 20 shown in FIGS. 11A, 11B is described hereinafter. FIG. 12 is a diagram showing variation in light transmittance in relation to a driving voltage of the ferroelectric liquid crystal device 20. In this case, switching of ferroelectric liquid crystals, that is, transfer of one stable state thereof to the other stable state occurs only in the case where a voltage is applied to the ferroelectric liquid crystals when the product between a pulse width value of the driving voltage and a pulse height value thereof becomes higher than a threshold value of the ferroelectric liquid crystal device.
In FIG. 12, the ferroelectric liquid crystal device 20 makes selection on either a non-transmitting (black display) state, which is the first stable state, or a transmitting (white display) state, which is the second stable state, depending on difference in polarity of the driving voltage thereof.
In this case, a voltage value at which the light transmittance starts to vary when the driving voltage is increased in the plus direction is assumed as V1 while a voltage value at which variation of the light transmittance is saturated is assumed as V2. Then, when the driving voltage is decreased, and the driving voltage reverse in polarity is increased in the minus direction, a voltage value at which the light transmittance starts to decrease is assumed as V3 while a voltage value at which variation of the light transmittance is saturated is assumed as V4. Thus, with the ferroelectric liquid crystal device 20, if the driving voltage not lower than the threshold value of the ferroelectric liquid crystal molecules (that is, a plus applied voltage not lower than V2) is applied thereto, the second stable state is selected while if the driving voltage not lower than the threshold value of the ferroelectric liquid crystal molecules, in reverse polarity, (that is, a minus applied voltage not lower than V4) is applied thereto, the first stable state is selected. Thereafter, even if the driving voltage is turned to 0V, the respective stable states are maintained owing to the memory effect.
As a result, if the first and second polarizing films 21a, 21b are disposed as shown in FIG. 11A, the ferroelectric liquid crystal device 20 in the second stable state turns into white display (the transmitting state), and the same in the first stable state turns into black display (the nontransmitting state). Further, if the arrangement of the first and second polarizing films 21a, 21b is changed, this will enable the ferroelectric liquid crystal device 20 in the second stable state to be in the black display (the nontransmitting state), and the same in the first stable state to be in the white display (the transmitting state), however, in the present description given hereunder, it is assumed that the ferroelectric liquid crystal device 20 in the second stable state is in the white display (the transmitting state), and the same in the first stable state is in the black display (the nontransmitting state).
Now, referring to FIG. 13, there is described the driving voltage for driving the ferroelectric liquid crystal device 20. In this case, it is assumed that the ferroelectric liquid crystal device 20 is driven by, for example, a time-sharing driving system, and the driving voltage is sequentially applied by time sharing to the plurality of the scanning electrodes 24, and the signal electrodes 25. FIG. 13 shows composite driving voltages applied to the scanning electrodes 24 of the ferroelectric liquid crystal device 20, and the signal electrodes 25 thereof, respectively, and waveforms for N=1 represent the composite driving voltage applied to pixels on a first length of the scanning electrodes 24 while waveforms for N=2 represent the composite driving voltage applied to pixels on a second length of the scanning electrodes 24. Furthermore, the driving voltages in the case of the black display are shown separately from the driving voltages in the case of the white display.
Now, with the composite driving voltage for N=1, there is provided a reset time period Rs for resetting the ferroelectric liquid crystal device to either one of the stable states before a time period for writing display data. The reset time period Rs has a switching time period Sw, and a non-switching time period NSw, and during the switching time period Sw, a bipolar pulse at +Vsw and −Vsw is applied while a voltage at 0V is applied during the non-switching time period NSw. Further, a voltage whose absolute value is sufficiently larger than the previously described threshold value (that is, V2, or V4) is selected as +Vsw or −Vsw. A waveform of the driving voltages during the reset time period Rs is defined as a reset pulse.
The ferroelectric liquid crystal device 20 can be reset to the first stable state (the black display) by applying the reset pulse thereto, but it is also possible to reset ferroelectric liquid crystal device 20 to the second stable state (the white display) by inverting the reset pulse.
Subsequently, a time period immediately after the reset time period Rs is a select time period Se for writing the display data, and in the case of the black display, the bipolar pulse at ±Vse1 which is a voltage not higher than the threshold value is applied while in the case of the white display, the bipolar pulse at ±Vse2 which is a voltage not lower than the threshold value is applied. After the select time period Se, a non-select time period NSe when the bipolar pulse at ±Vnse, which is a voltage not higher than the threshold value, is applied, will continue until completion of scanning.
In the case of the composite driving voltage at N=2, a switching time period Sw of a reset time period Rs is the same in timing as the switching time period Sw for the composite driving voltage at N=1, and a select time period Se lags behind in timing by one scanning electrode. Consequently, a non-switching time period NSw varies in length by the scanning electrode, gradually increasing in length as the ordinal number of the scanning electrode increases. That is, the ferroelectric liquid crystal device 20 is driven by concurrently applying the bipolar pulse to all the pixels during the switching time period Sw of the reset time period Rs, and subsequent writing of display data to the respective scanning electrodes 24 is executed by applying the voltage after causing the select time period Se to lag behind in timing by each of the plurality of the scanning electrodes 24.
With the ferroelectric liquid crystal device, and the method for driving the same, disclosed in the Patent Document previously described, the non-switching time period NSw when a voltage 0V is applied is provided within the reset time period Rs, thereby rendering it possible to control variation in the threshold value dependent on the display state of liquid crystals, so that excellent display can be effected by controlling the sticking phenomenon.
Further, since the threshold value of a ferroelectric liquid crystal device has temperature characteristics, a method for driving the ferroelectric liquid crystal device, whereby the temperature characteristics thereof is compensated for, has been disclosed in, for example, JP 2616496 B (p. 2, FIG. 8, and so forth).
The conventional driving method for compensating for the temperature characteristics of the ferroelectric liquid crystal device is described hereinafter with reference to the accompanying drawings. FIG. 14 is a graph showing temperature compensation characteristics varying a pulse width of a driving voltage according to temperature in order to compensate for the temperature characteristics of the ferroelectric liquid crystal device. In this case, a threshold value at which the ferroelectric liquid crystal device undergoes switching has characteristics that the threshold value is small in high-temperature state, but becomes larger as temperature becomes lower.
Accordingly, as shown in the figure, the driving voltage for driving the ferroelectric liquid crystal device needs to have a large pulse width at a low temperature although a small pulse width may be sufficient at a high temperature. For example, the ferroelectric liquid crystal device can be driven with the driving voltage having a pulse width of several hundred μS at 60° C., however, at −10° C., the driving voltage having a pulse width of about 9000 μS will be required. Further, in order to effect temperature compensation for the threshold value, it is preferable to vary not only the pulse width of the driving voltage but also the pulse height of the driving voltage according to temperature, thereby effecting the temperature compensation in respect of both the pulse height and the pulse height. Thus, by varying the driving voltage according to variation in temperature, the ferroelectric liquid crystal device can be stably operated regardless of temperature.
However, it has been verified by experiment that the threshold value at which the ferroelectric liquid crystal device undergoes switching is affected by humidity as well while having the temperature characteristics.
With reference to the accompanying drawings, humidity characteristics of the ferroelectric liquid crystal device are described hereinafter. FIG. 15 is a view showing an example of the humidity characteristics of the threshold value of the ferroelectric liquid crystal device when an ambient temperature is at −10° C. and at 0° C., respectively. In the figure, the x-axis indicates an ambient humidity of the ferroelectric liquid crystal device, and the y-axis indicates a threshold value Pth of a pulse width of the driving voltage for causing the ferroelectric liquid crystal device to undergo switching.
Herein, a threshold value P1 indicated by a solid line represents the threshold value of a pulse width for causing the switching when a select voltage (5V) is applied at −10° C. while a threshold value P2 indicated by another solid line represents the threshold value of a pulse width for causing the switching when a non-select voltage (1.66 V) is applied at −10° C. Further, a threshold value P3 indicated by a broken line represents the threshold value of a pulse width for causing the switching when the select voltage (5V) is applied at 0° C. while a threshold value P4 indicated by another broken line represents the threshold value of a pulse width for causing the switching when the non-select voltage (1.66 V) is applied at 0° C.
In this case, the select voltage refers to the driving voltage applied during the select time period Se as described with reference to the composite driving voltages shown in FIG. 13, and the non-select voltage refers to the driving voltage applied during the non-select time period NSe.
Now, if attention is focused on the threshold values at −10° C., that is, P1, P2, it is shown that in high-humidity state, the threshold value P1 upon application of the select voltage is low in value while the threshold value P2 upon application of the non-select voltage is high in value. Accordingly, even if the select voltage (5V) is applied with an intermediate pulse width between the respective pulse widths of the threshold values P1, P2 to thereby, for example, invert display from black to white, and subsequently, the non-select voltage (1.66 V) with the same pulse width is applied in order to rewrite display content, the ferroelectric liquid crystal device will not undergo switching at the non-select voltage since the threshold value P2 at the non-select voltage is high in value, so that it is possible to secure a large operational margin.
However, in the case of a drop in humidity, as shown in the figure, as the humidity becomes lower, so does higher the threshold value P1 while as the humidity becomes lower, so does lower the threshold value P2. Then, the threshold values P1, and P2 intersects each other under a condition of certain humidity, and upon a further drop in humidity, relative orientations of the threshold values P1, P2 are reversed. Assuming that a point where the threshold values P1, and P2 intersects each other is referred to as an intersection K1, a region on the left side of the intersection K1, in the figure, higher in humidity than the intersection K1, is a normal operation region with a given operational margin since the threshold value P2 at the non-select voltage is higher than the threshold value P1 at the select voltage.
However, a region on the right side of the intersection K1, in the figure, lower in humidity than the intersection K1, is a region where a relationship between the threshold value P1 at the select voltage and the threshold value P2 at the non-select voltage is reversed, so that normal switching of the ferroelectric liquid crystal device cannot be implemented. This region is therefore defined as an abnormal operation region. More specifically, even if the ferroelectric liquid crystal device is caused to undergo switching at the select voltage, the ferroelectric liquid crystal device ends up undergoing switching again at the non-select voltage in the abnormal operation region because the threshold value P2 at the non-select voltage is lower than the threshold value P1 at the select voltage, so that a normal operation cannot be executed. Furthermore, in the vicinity of the boundary of the abnormal operation region even though not inside the abnormal operation region, the threshold value P1 at the select voltage is so close in value to the threshold value P2 at the non-select voltage, so that there is the risk that the operational margin decreases, thereby causing occurrence of an unstable operation.
Next, if attention is focused on the threshold values at 0° C., that is, P3, P4 it is shown that the influence of humidity is lessened, and an intersection K2 where the threshold values P3, P4 intersects each other is shifted to a point very low in humidity, so that most of an operation region falls within the normal operation region. Further, if temperature is not lower than 0° C. (not shown), the influence of humidity becomes small, and operation will be in the normal operation region regardless of a humidity level.
It has turned out from the foregoing that the threshold value of the ferroelectric liquid crystal device largely affected by humidity under a condition of the ambient temperature not higher than 0° C., and that there is a good possibility of the ferroelectric liquid crystal device finding itself within the abnormal operation region when the temperature is not higher than 0° C., and the humidity is low. Further, it is confirmed by experiment that the abnormal operation region is prone to occur when a condition of low temperature and low humidity is sustained for a given time period.
Because of a phenomenon described as above, it becomes necessary to compensate for not only the temperature characteristics but also the humidity characteristics in order to stably operate the ferroelectric liquid crystal device in a wide temperature range.
However, with the ferroelectric liquid crystal device disclosed in International Publication No. WO 00/23848 A1 as previously described, compensation for temperature and compensation for humidity have not been taken into consideration. This poses a problem in that there exists the risk of the fertoelectric liquid crystal device undergoing erroneous operation under a condition of low temperature and low humidity.
Further, with the method for driving the ferroelectric liquid crystal device disclosed in JP 2616496 B as previously described, the compensation for temperature has been taken into consideration, however, compensation for humidity has not been taken into consideration, which similarly poses the problem in that there exists the risk of the ferroelectric liquid crystal device undergoing erroneous operation under the condition of low temperature and low humidity.
Next, referring to FIGS. 16A to 17B, a conjecture is made on the principle behind the phenomenon that the ferroelectric liquid crystal device finds itself in the abnormal operation region in the case of low temperature, and low humidity. Now, FIG. 16A is a schematic view of a model representing polarities of ions in a pixel and orientation of spontaneous polarization exhibited by liquid crystal molecules, in the case where ferroelectric liquid crystals are sandwiched between a pair of glass substrates 40a, 40b, in which a black circle indicates a plus ion, and a white circle a minus ions while arrows between the black circles, and the white circles indicate the orientation of the spontaneous polarization. In FIG. 16A, an assumption is made such that a ferroelectric liquid crystal device, in a state where it is placed in an environment of low temperature and low humidity, is turned into white display representing one of the stable states when a reset pulse is applied thereto.
Since the ferroelectric liquid crystal molecules have the spontaneous polarization at this point in time, residual ionizable impurities that are present in the ferroelectric liquid crystals are attracted to the surfaces of respective alignment layers (not shown) formed on the surfaces of the glass substrates 40a, 40b, opposed to each other, by the agency of an internal electric field Eps occurring within a cell, due to the spontaneous polarization, so that the minus ions exist on a side of the cell, adjacent to the glass substrate 40a, and the plus ions exist on a side of the cell, adjacent to the glass substrate 40b. The minus ions and the plus ions create an ionic electric field Eion in such a direction as to cancel out the internal electric field Eps.
Now, in the case where SiO is used for the alignment layers, impurity ions in the liquid crystals, in a state of low temperature and low humidity, are prone to be attracted by SiO in the alignment layers, and furthermore, the impurity ions attracted to the respective alignment layers (namely, the minus ions and the plus ions) in the state of low temperature and low humidity, in particular, are in a state of extreme difficulty in their migration, so that even if the ferroelectric liquid crystal device undergoes switching by the agency of the external electric field, and the spontaneous polarization of the liquid crystal molecules is thereby inverted, the impurity ions do not follow up the liquid crystal molecules, holding a preceding state thereof.
Next, FIG. 16B is a schematic view showing a state where the ferroelectric liquid crystal device is turned into black display from a state where the white display is maintained in FIG. 16A, due to inversion of the spontaneous polarization of the liquid crystal molecules, occurring upon application of a select voltage Eout 1. In this state, the internal electric field Eps occurring due to the spontaneous polarization is inverted, however, since the ferroelectric liquid crystal device is in the state of low temperature and low humidity, the impurity ions do not follow up the liquid crystal molecules as previously described, so that there will be no change in the direction of the ionic electric field Eion.
Next, FIG. 17A is a schematic view showing a state where the impurity ions do not follow up the liquid crystal molecules, and the direction of the ionic electric field Eion is unchanged even if the black display state of the ferroelectric liquid crystal device due to inversion of the spontaneous polarization of the liquid crystal molecules continues, after completion of the application of the select voltage Eout 1. The ionic electric field Eion in this case acts so as to re-invert the spontaneous polarization of the liquid crystal molecules (to revert to a preceding state), however, because the ionic electric field Eion is not so large in intensity, the ionic electric field Eion is unable to cause instantaneous inversion of the liquid crystal molecules.
Then, FIG. 17B is a schematic view showing a state where a non-select voltage Eout 2 is applied to the liquid crystal molecules. Herein, the non-select voltage Eout 2 is a voltage lower than the select voltage Eout 1 as previously described, however, in this case, the non-select voltage Eout 2 of an electric field identical in direction to the ionic electric field Eion acts on the liquid crystal molecules, whereupon the spontaneous polarization of the liquid crystal molecules is inverted, thereby causing the ferroelectric liquid crystal device to revert to the white display. Consequently, the liquid crystal molecules will be inverted at the non-select voltage due to the effect of the ionic electric field Eion, although the non-select voltage inherently does not exceeds a voltage at the threshold value of the liquid crystal molecules, and switching does not occur at the non-select voltage.
Further, the lower temperature and humidity become, the more the impurity ions are attracted to the respective alignment layers, thereby causing the ionic electric field Eion to increase in intensity, so that the lower the temperature and the humidity become, the smaller becomes the threshold value Pth for causing the liquid crystal molecules to undergo switching, thereby increasing a possibility of erroneous operation being caused by a voltage applied during the non-select time period.
To sum up the conjecture described in the foregoing, the impurity ions are more prone to be attracted to the respective alignment layers at low temperature, and low humidity. Consequently, the ionic electric field Eion increases in intensity. Accordingly, the threshold value Pth of the pulse width during the non-select time period becomes smaller. For this reason, the threshold value Pth necessary for switching from one state such as a reset state to the other state becomes greater, but the threshold value Pth necessary for reverting to a preceding state (in this case, the voltage during the non-select time period) becomes smaller. It is presumed that owing to the above-described reason, the abnormal operation region occurs at low temperature, and low humidity.
Further, in the case where an inorganic vapor-deposited film such as an SiOx film, and so forth, susceptible to the influence of humidity, is adopted for the alignment layers, the phenomenon described as above occurs in a pronounced way. Accordingly, even in the case where material other than smectic liquid crystal such as the ferroelectric liquid crystal, and so forth is used as a liquid crystal material, it is presumed that a similar phenomenon will occur because of large influence of humidity when the inorganic vapor-deposited film is adopted for the alignment layers.